#!/usr/bin/python2

def binary_search(list, item):
    low = 0
    high = len(list) - 1
    while low <= high:
        mid = (low +high)/2
        guess = list[mid]
        if guess  == item:
            return mid
        if guess < item:
            low = mid + 1
        else:
            high = mid -1    
    return None

my_list = [1, 3, 5, 7, 9]
print binary_search(my_list, 3)
print binary_search(my_list, -1)


